The inner shell of a storage tank for liquefied natural gas (“LNG”) is normally made from 5-9% nickel steel due to its excellent combination of high strength and satisfactory impact toughness at very low temperature, e.g., −196° C.
When such a tank is constructed on-site, the individual steel sheets forming the tank interior are normally joined by welding. Horizontally-oriented seams are primarily welded using automatic welding processes, using ERNiCrMo3 and ERNiCrMo4 submerged arc flux/wire combinations.
In contrast, most vertically-oriented seams are welded manually by shielded metal arc welding (SMAW), which is also known as “stick” welding. This is because the vertical welds that are produced by current automatic welding processes are less than ideal in operability, strength, impact toughness at −196° C., susceptibility to welding hot cracking or bead shape.
In this regard, weld beads that are produced when adjacent sheets of 5-9% nickel steel are welded are desirably as flat as possible in the sense of being neither concave nor convex when viewed in transverse cross section. In addition, they should also be as uniform as possible from region to region, i.e., along their entire lengths. Vertical welds in 5-9% nickel steel workpieces are normally made manually, because of the flexibility this type of welding allows. For example, electrode angle and location inside the seam, welding speed, dwell time, etc. can be varied, allowing an experienced welder to produce a “perfect” weld bead along the entire length of the seam. Automatic welding machines do not have this flexibility. Furthermore, the stick electrode normally used for manual welding of 5-9% nickel steel, ENiCrMo-6, is significantly different from the existing flux cored welding electrodes used by automatic welding processes, both in terms of the metal from which it is made as well as the flux. This difference is also believed to play a role in the superiority of the vertical welds produced by manual welding.